Strengthened films and method for producing same

ABSTRACT

A film is provided which is highly suitable for use in magnetic tapes such as for tape recording. The film provides polyethylene1,2-diphenoxyethane-4,4&#39;&#39;-dicarboxylate tape with characteristics heretofore not possible, including an increased longitudinal F-5 value and an improved longitudinal tensile strength. More specifically, in accordance with the invention, there is provided a biaxially stretched film suitable for use as a tape for magnetic recording which is a biaxially stretched film of polyethylene-1,2-diphenoxyethane-4,4&#39;&#39;-dicarboxylate having a longitudinal F-5 value of 20 Kg/mm2 to 50 Kg/mm2, a longitudinal tensile strength of 30 Kg/mm2 to 65 Kg/mm2, a longitudinal initial modulus of elasticity of 800 Kg/mm2 to 1,500 Kg/mm2, a transversal tensile strength of 8 Kg/mm2 to 20 Kg/mm2 and unevenness of thickness of 5% or less, which is produced by first stretching a substantially amorphous polyethylene-1,2diphenoxyethane-4,4&#39;&#39;-dicarboxylate layer whereby there is produced an orientation at least in the direction perpendicular to the extrusion direction and a film density after stretching of not greater than 1.335 in said first stretching, and thereafter subjecting the thus-treaded film to a second stretching step in the extrusion direction at a temperature of 100* to 200*C to yield a total stretching ratio of 5 to 8 times the original length in said second stretching step.

United States Patent 1 Kohno et al.

[ STRENGTHENED FILMS AND METHOD FOR PRODUCING SAME [75] Inventors: Mitsuo Kohno, Moriyama; Minoru Nomura, Yokohama; Akio Shibazaki, Tokyo; Takeo Yuasa, Asaka; Yoshihiko Mutoh, Moriyama, all of Japan [73] Assignee: Asahi Kasei Kogyo Kabushiki Kaisha, Osaka, Japan [22] Filed: Oct. 4, 1973 [21] Appl. No.: 403,486

[30] Foreign Application Priority Data Oct. 9, 1972 Japan 47-100610 [52] US. Cl 260/47 C; 428/336; 428/910; 264/289 [51] lnt.Cl.....B29D 7/24; B3213 27/36; C0 8G 63/02 [58] Field of Search 161/402, 165; 264/288, 264/289, 290 R, 290 J, 210 R; 260/47 C [56] References Cited UNITED STATES PATENTS I 2,928,132 3/1960 Richards 264/289 2,968,065 1/1961 Gronholz.............,.............. 264/289 3,432,591 3/1969 Heffelfinger 264/289 3,574,046 4/1971 Buteuxmue... 161/402 3,627,625 12/1971 Jarrett 161/402 3,631,899 1/1972 Erickson. 161/402 3,700,540 10/1972 Buteux 161/402 3,816,368 6/1974 Kiichiro 161/165 6 E i 3 l-330- LL O [451 Aug. 26, 1975 Primary Examiner-George F. Lesmes Assistant Examiner-Ellis P. Robinson Attorney, Agent, or Firm-Armstrong, Nikaido & Wegner [57] ABSTRACT A film is provided which is highly suitable for use in magnetic tapes such as for tape recording. The film provides polyethylene-1,2-diphen0xyethane-4A dicarboxylate tape with characteristics heretofore not possible, including an increased longitudinal F-S value and an improved longitudinal tensile strength. More specifically, in accordance with the invention, there is provided a biaxially stretched film suitable for use as a tape for magnetic recording which is a biaxially stretched film of polyethylene-l,2-diphenoxyethane- 4,4'-dicarboxylate having a longitudinal F- value of Kg/mm to Kglmm a longitudinal tensile strength of 30 Kg/mm to Kg/mm a longitudinal initial modulus of elasticity of 800 Kg/rnm to 1,500 Kg/mm, a transversal tensile strength of 8 Kg/mm" to 20 Kg/mm and unevenness of thickness of 5% or less, which is produced by first stretching a substantially amorphous polyethylene l ,2-diphenoxyethane-4,4'- dicarboxylate layer whereby there is produced an orientation at least in the direction perpendicular to the extrusion direction and a film density after stretching of not greater than 1.335 in said first stretching, and thereafter subjecting the thus-treaded film to a second stretching step in the extrusion direction at a temperature of to 200C to yield a total stretching ratio of 5 to 8 times the original length in said second stretching step.

6 Claims, 1 Drawing Figure loo u'o lo STRENGTHENED FILMS AND METHOD FOR PRODUCING SAME DESCRIPTION OF THE lNVFNTlON This invention relates to strengthened films and a method for producing same. More particularly, it relates to biaxially oriented polyethylene-1,2- diphenoxyethane-4,4'-dicarboxylate films having mechanical properties which show great strides of im' provement in the extrusion direction (hereinafter referred to as longitudinal direction) and also sufficient strength in the direction perpendicular to the extrusion direction (hereinafter referred to as transversal direction) and a method for producing same,

Heretofore films having an extremely high grade of longitudinal mechanical properties and a sufficient transversal strength have been wanted in various application fields, as a so-called strengthened film. Such films have been prepared mainly from polyethylene terephthalate but recently in the use of those films, it has been earnestly requested to impart to films, higher modulus of elasticity, higher strength and dimensional stability at the time of heating. If these properties are improved, lightness and smallness of packages, and extension of service time for continuous use are realized in the fields of. e.g., magnetic recording tapes, sound recording tapes, type writer ribbons, etc., and commercial values of products will be improved with a great stride. This is the reason of the above-mentioned demand.

conventionally known strengthened polyethylene terephthalate film having superior properties in one direction had only a longitudinal F- value ofat the highest, l8 2U Kg/mm". Further, it shows a poor dimensional stability at the time of heating and an initial modulus of elasticity of at the highest, about 750 Kg/mm Accordingly, it is not sufficient for the abovementioncd demand.

it is an object of the present invention to provide films having a high modulus of elasticity suitable as base films for magnetic tapes, type writer ribbons, etc.

lt is another object of the present invention to provide films having a high grade of dimensional stability and capable of reducing their thickness down to a practical extent.

it is a further object of the present invention to provide a method for producing same.

We have been studying on the films which can overcome the drawbacks of the above-mentioned conventional films and as a result of such study, we have found that a novel highly strengthened film can be prepared from polyethylenel .2-diphenoxyethane-4,4 dicarboxylate which is a kind of polyesterether, by the biaxial stretching under specified conditions, and completed the present invention.

According to the present invention, biaxially stretched, longitudinally strengthened polyethylenel,2-diphenoxyethane 4,4'-dicarboxylate films having a longitudinal modulus of elasticity of 800 Kg/mm 1,500 Kg/mm", preferably 900 Kg/mm l,5t)() Kg/mm". a longitudinal F-5 value of It) Kg/mm" 5O Rig/mm", preferably 23 Kg/mm 5t) Kg/mm, more preferably Kg/mm- 50 Kg/mm a longitudinal tensile strength of 30 Kg/mm" 65 Kg/mm preferably Kg/mm" as Kg/mm a transversal tensile strength of 8 Kg/mm 2U Kg/mm, preferably l0 Kg/mm" 20 Kg/mm a thickness of l ZUU a, an unevenness of thickness of 5% of the original thickness or less and a density of L340 1.400, can be obtained.

As for biaxially stretched films of polyethylene-1,2- diphenoxyethane-4,4'-dicarboxylate which have been known heretofore, there is-disclosed in British Pat. No. 1,171,685 a preparation method for so-called balanced films which are stretched longitudinally and transversally to two or more times the original length so as to give substantially equal tensile strength both in the longitudinal and transversal directions. The method of this patent literature is directed to the production of a film having physical properties equal in the longitudinal and transversal directions as above-mentioned and not other than that. It was, however, impossible according to this method to stretch to 4 times the original length longitudinally and transversally, and resultant films have only a maximum longitudinal yield strength of at the highest 18 Kg/mm and a tensile strength to an extent of 25 Kg/mm Further, British Pat. No. l,l 36,l33, discloses a method for producing uniaxially oriented, longitudinally strengthened films having no tendency transversal fibrillation in which an amorphous film of same polymer is heated and after being crystallized it is longitudinally stretched. According to this method it is true that those having a longitudinal initial modulus of elasticity of more than 1,500 Kg/mm can be obtained, but as a matter of fact, the transversal strength of products is extremely low and according to the example illustrated, it is about 3 Kg/mm as expressed by yield stress. Moreover, it is common sense for a person skilled in the art that unevenness of thickness in the trasversal direction of product films is large and hence the products cannot be called as high grade strenghtened film at the least and entirely different from the biaxially stretched films of the present invention.

As above mentioned, strengthened films of the polymer of the present invention prepared according to the known process are only those having longitudinal initial modulus of elasticity, longitudinal F5 value, longitudinal tensile strength, etc., which are hardly different from the values of strengthened films consisting of already known polyethylene terephthalate or those having important defects in unevenness of transversal thickness, mechanical properties, etc.

Whereas we have investigated the characteristic properties of polyethylene-l ,2diphenoxyethane-4,4- dicarboxylate and earnestly studied the processing conditions of stretching which were most suitable to this polymer, and found that the above-mentioned strengthened films having longitudinal initial modulus of elasticity, longitudinal F-S value and longitudinal tensile strength which are unexpectably increased and also sufficient transversal tensile strength, are produced by the process hereinafter described. The films of the present invention is also characterized by an extremely small unevenness of thickness in the transversal direction so small as 571 or less. Uniaxially strengthened films have been known in the abovementioned British Pat. No. l,l36, l 33, but the films obtained according to the method of this patent has defect in relatively large unevenness of thickness such as 10% or more. Uniaxially strenthened films having a sufficient strength also in the transversal direction and a small unevenness of thickness such as 57( or less have been obtained for the first time according to the method of the present inven tion.

The films of the present invention are produced by stretching substantially amorphous polyethylene-1.2- diphenoxyethane-4.4'-dicarboxylate film extruded from a die so as to form orientation at least in the direction perpendicular to the extrusion direction and to give a density of the resulting stretched film of 1.335 or less in the first step stretching. and further stretching in the extrusion direction at a temperature of 100 200C to 5 8 times the original length in the total stretching ratio in the second step stretching. and if necessary by heat-setting the resultant stretched films at a temperature of 150 220C. The production method of the stregthened film of the present invention includes two stretching steps: In the first step. a film is stretched at least in the transversal direction to give a film having a density of 1.335 or less. and in the second step. the resultant film is stretched at l 200C in the longitudinal direction to give a total longitudinal stretching ratio of 8 times the original length. In other words the first step stretching includes a case of transversal stretching alone. and other cases where transversal plus longitudinal stretchings are carried out simultaneously or stepwise in any order. so long as stretching is carried out at least in the transversal direction to give a film having a density of L335 or less. and the second step stretching includes a case where longitudinal stretching is carried out one time. and other cases where longitudinal stretching is carried out more than one time. so long as a total longitudinal stretching ratio of 5 8 times is obtained. In each of these cases. heat-setting is carried out if it is necessary. In the method of the present invention. unlike conventional films. films which show relatively small heat shrinkage and good dimensional stability can be obtained even when heat-setting is not applied.

The polyethylenel .2diphenoxyethane-4.4 dicarboxylate useful in the present invention can be produced by the condensation of ethylene glycol with l.2-bis-( p-carboxyphenoxy)ethane or by the ester interchange reaction of preferably ethylene glycol and an ester-formable derivative of l.2-bis-( p-carboxyphenoxy)ethane such as l.2-bis-(p-carbomethoxy phenoxylethane and the polycondensation of the resultant reaction product.

As for catalysts useful in the ester-interchange reac tion at that time. manganese compounds such as conventional manganese acetate. etc.. and calcium compounds such as calcium acetate are common.

As for catalysts useful in the polycondensation reac' tion. antimony compounds such as antimony trioxidc. etc.. germanium compounds such as germanium dioxide. etc.. are common.

In the practice of the present invention. it is possible to use polyethylenel .Z-diphenoxycthane-4.4 dicarboxylate copolymerized with other component in an amount up to mol 7: or a mixture ofthese copolymersv Copolymerizable monomers include one kind or more of glycol selected from the group consisting of propylene glycol. diethylcne glycol. triethylcne glycol. tctraethylcne glycol. etc.. or one kind or more of dicar boxylic acid selected from the group consisting of phthalic acid. bi-benzoic acid. adipic acid. sebacic acid.

hexahydrotcrephthalic acid. naphthalcneilidicarboxylic acid. etc.

The substantially amorphous polyethylene-l .2-

diphenoxycthane-4.4-dicarboxylatc useful in the prestill ent invention can be obtained by quenching the polymer extruded from a slot die to a temperature of room temperature to 70C. preferably C or lower. Uniaxial or biaxial stretching of resultant films by way ofconventional process brings about great difficulty due to the specific nature of its crystallization behavior. Namely. since the glass transition temperature of polyethylene-l .2-diphenoxyethane-4.4'-dicarboxylate is about C and since the crystallization starting temperature of heated amorphous material is about C. the optimum stretching temperature which has been heretofore considered to be acceptable from common sense is in so extremely narrow range such as 70 74C. There is also a process which. contrary to common sense. is carried out at a further higher temperature. but at such a temperature. crystallization advances by heating and reduces stretching property. For example, in the uniaxial stretching of the abovementioned British Pat. No. l.l 36.] 33. stretching ratio greater than 5 times was difficult and in the biaxial stretching of British Pat. No. l.l7 l .685. stretching greater than 4 times in both the directions was difficult. As a result of our study on the process of biaxial stretching. we have found that longitudinal stretching to 5 times or more and as maximum up to 8 times is possible by the process in which substantially amorphous layer is stretched at least in the direction perpendicular to the extrusion direction at a relatively low temperature but at a temperature at which a sufficient orientation occurs. to give an at least uniaxially stretched film under condition of suppressed crystallization as much as possible although birefringence is formed therein. which is further stretched on heating rapidly in the longitudinal direction.

The production process of the films of the present invention will be described in detail.

In order to bring the substantially amorphous polyethylene-l.2-diphenoxyethane-4.4'-dicarboxylate films in the state oriented at least in the direction perpendicular to the extrusion direction. it is uniaxially stretched in the direction perpendicular to the extrusion direction in the first stretching or it is biaxially stretched in the direction of extrusion and the direction pcrpendic ular to the extrusion direction either simultaneously or stepwise. In the uniaxial stretching in the direction perpendicular to the extrusion. as well as in the biaxial stretching. the optimum temperature for stretching. although varied according to stretching ratio. is |00C or lower. preferably C or lower. However. it is preferable to avoid a temperature lower than 70C. because the production of uniformly stretched films at a speed suitable to the actual production is difficult. The stretching ratio at this time is 2 4 times in the extrusion direction and in the direction perpendicular to the extrusion direction. With the increase of stretching ratio. crystallization due to orientation is promoted and when stretching ratio is increased. it is preferable to carry out stretching at a lower temperature.

It is a generally accepted manner to express the extent of crystallization by way of density. The density of pe rfcctly amorphous polyethylene-l .2- diphenoxyethane-4A'-dicarboxylate is L303 and that of perfectly crystalline material is L450. 'Ihc films which are oriented at least in the direction perpendicular to the extrusion direction in the first step stretching (important requirement for producing the films of the present invention) should have a density in the range of 1.335 to 1.315 and most preferable density is in the range of 1.330 1.320.

The above-mentioned relationship can be described referring to attached drawing wherein the relationship between stretching temperature and stretching ratio and density of film is shown as a graph. and a symbol [:1 I] shows a uniaxial stretching with 3.5 times of stretching ratio. 0 0 shows a uniaxial stretching with 3.0 times of stretching ratio. A A shows a uniaxial stretching with 2.0 times of stretching ratio and shows a biaxial stretching with stretching ratios of 3.0 times X 3.0 times. Thus the FIGURE shows the relation of the change of the density of these films relative to stretching temperature and stretching ratio at the time when substantially amorphous films having a density of 1.306 are uniaxially stretched at a fixed width by means of a biaxial stretching apparatus (manufactured by lwamoto Seisakusho) and also at the time when they are biaxially stretched in the same apparatus.

As evident from this FIGURE. stretching temperatures in the optimum range corresponding to a density of 1.3 are 86C for uniaxial stretching with 3 times of stretching ratio. 92C for uniaxial stretching with 2 times of stretching ratio and 92C for biaxial stretching with 3 times X 3 times of stretching ratios.

With regard to the stretching in the extrusion direction. i.e.. the second step stretching. stretching is carried out to 5 times 8 times preferably 6 8 times the original length in total in the extrusion direction at a high temperature of C 200C. 1t is a fact which cannot he anticipated from the conventional art that only the films which have been maintained at a specified density under the above-mentioned conditions and oriented at least in the direction perpendicular to the extrusion direction can be stretched with such a high stretching ratio as high as 5 times or more in total in the extrusion direction in the second step stretching of extrusion direction.

The stretching temperature in the second step stretching should be a temperature of 100C or higher. preferably in the range of C 180C. If it excels 200C. molecular flow of polymer occurs to make stretching effectiveness poor. and hence the excelling temperatures are not preferable. The higher the stretching temperature is. the more the crystallization degree is increased. and the less the heat shrinkage after stretching is. If a suitable stretching temperature is selected. there is an advantage that heat-setting can be omitted after stretching or even heat-setting carried out under extremely gentle condition serves the pur pose. Particularly, if the second step stretching is carried out in two or more stages at a higher temperature in the above-mentioned range. the above-mentionecl effect can be attained much more easily.

The second step stretching can be carried out by a customary procedure which relies on the pass between a slow roll set and a fast roll set. It is preferable to make the stretching region of film as narrow as possible. If the diameters of rolls are narrowed. and the free path line between two roll sets is narrowed very much. the above-mentioned requirement is satisfied. but there is a limitation for the above-mentioned problem from the strength of apparatus and other restrictive conditions. There is another method. as alternative. wherein. in place of the method of heating slow and fast rolls and thereby maintaining the stretching temperature. an ex (ill tremcly narrow width range of film. c.g.. 50 min or less in the direction of length is heated by the irradiation of infrared ray or the likc and the stretching point is fixed almost in that range. This is one of the most advantageous method which enables to perform stabilized and uniform stretching in the point that adhesion and sticking of film onto rolls at high temperatures do not occur. Since an accurate control is necessary for stretching temperature. a controlling method carried out by way of the control of the capacity of infrared heater while measuring the actual temperature of the film at the stretching point irradiated by the infrared heater. by means of infrared thermometer on sale or the like. is employed.

Since the films thus obtained show small heat shrinkage. they can be used. as they are. but if desired. there will be no harm in heat-setting at a temperature of C or higher. preferably in the range of 180 220C. By the heat-setting. films having extremely low shrinkability can be obtained. For example. films superior in dimensional stability which show shrinkage of 1 4%. preferably 1 3'71. after the heat treatment at 150C for 2 hours can be obtained.

The polymers useful in the films of the present invention are polyethylene-l .Z-diphenoxyet ham-4.4- dicarboxylate and its copolymer. it is preferable that the molecular weight of the polymer corresponding to an intrinsic viscosity [1 as measured in 17 solution in o-chlorophenol at 35C is 0.5 or more. Too high a molecular weight is not preferable because of expensive polymer cost. Too low a molecular weight causes trouble in film-forming property.

So long as the property of film is not injured. admixing of various kinds of additives to the polymers of the present invention can be carried out arbitrarily. For example. slip-improving agent (lubricant) such as kaoline. Si0 TiO- ctc.. pigments antistatic agents. crystallization-nucleating agents. light stabilizers. etc. can be added Without producing any harmful effect.

Another characteristic feature of the films of the present invention is casiness for forming thin foil when compared with films consisting of known polyesters such as polyethylene terephthalate or the like. Even under conditions for sufficiently elevating strength. thin foil can be formed with good productivity. As described in the beginning of the specification. thin foil forming due to strenghthening can bring about prefera ble effect upon the attempt to turn commercial products such as magnetic tapes. etc.. to lighter weight products and longer products.

In order to describe the nature of the present invention more fully. following examples are offered relating to the process for producing the films of the present invention but it should not be construed to be limitative to the present invention.

Tensile strength. F-S value. elongation at break. initial modulus of elasticity were measured according to J1S-C-23l8-1966.

EXAMPLE 1 Polyethylene 1 .2-diphenoxyethane-4.4'carboxylatc having an intrinsic viscosity of 0.68 as measured in o' chlorophcnol (as 1% solution) at 35C. was extruded from T-die at 280C onto a casting roll cooled to 60C to give a substantially amorphous film having a thickness of 200 [1. and a density of 1.308. The film thus obtained was at first stretched to 3 times the original width with a given width in the perpendicular direction to the extruding direction by means of an experimental apparatus manufactured by lwamoto Seisakusho, Japan (in the first step stretching). The density of the resulting film is shown in Table l. The film was then EXAMPLE 2 Dried chips of polyethylene-l.ldiphenoxycthane- 4.4'-dicarbox vlate were extruded from a T-die at stretched by means f halting n i h pcrpcmlicutur 280C onto a casting roll cooled to 60C to give a sub. direction to the foregoing stretching direction with a stantially amorphous film having a thickness of l()() a, free width (second step stretching). The maximum a density of L304 and an intrinsic viscosity of().(14 as stretching ratios at break at a heating roll temperature measured in o-chlorophenol (as l/: solution) at 35C. of 140C are also shown in Table l. I The resulting film was continuously led to a tentcr ap- As seen from the 'lahle, when the density exceeds paratus [laterally stretching machine). where it was L335, stretching to 5 times or more becomes impossistretched to various stretching ratios in the direction ble. perpendicular to the extruding direction at various pre- T'lhic l heating stretching temperatures shown in Table 2 (first step stretching 1. Next. the clip-holded parts at both the edges, of the resulting film were trimmed and thereaf- First step stretching Second step stretching g v I I Ratio Temperature Dcnsit Temperature Maximum U mli'hlna arranged m the extrudt'l ing direction. provided with an infrared heater between I'll It) a slower set and a faster set of rolls. where the film was 3 74(' 1 324 140C 71 211 stretched at various temperatures and with various ra- :1 tios shown in Table 2 (second step stretching). Each 1 1 I g, 1 33| 1411 resultmg film was heat-set at 2l()( if necessary. The i physical properties of the films obtained under these conditions are also shown in Table 2.

Table 2 TD MD lhys' al properties of biaxially stretched film stretching stretching (Values in parentheses are those after heat set) conditions conditions Tcm- Densit Tem- Thickl nc\ enl)ensit l-'-5 \aluc 'lcnsile Elongation Heat per- Ratio of uniper- Ratio ncss ness of strength at break shrinkage aturc axially atun: thick- 1 Kg/mm (Kg/111111 (J (11' 1 stretchness ed film No. 1(') 1 (i 1" lg/cm) (Kg/mm") Ml) 'I'D MI) TD Ml) TD MD ID 1 711 3.11 1.3211 1511 7.3 4.3 2 s 1347 1111111 3 1 7.11 411 11 11 1111 7.7 11.11 2 so 3.11 1.327 1511 7.5 4.2 2.x 1.34s 111111 44 s.2 5: 12 111 77 7.4 11.11

11.353 1|(IX11) 1411 1 1.11 (481 11-11 111) 1821 (I5) (L11 3 1115 3.11 1.3311 1511 4.2 7. 11.2 1.3411 7s11 17 s.1 24 11 1 42 11.2 7.11 -1 so 211 1.323 1511 1-.11 s.1 3.11 1.345 111411 32 7.4 41 111 111 1-111 11.11 11.11 5 so 2.5 1.325 1511 21.2 4 s 2. 1 1.347 11211 -12 7.5 111 1 11.11 7.4

(1.354) (I080) (38) (8.3) (48) (ll) H2) 182) (L2) (0.9) 11 so 3.5 1.331 1511 5.x 4. 1 1 s 1.34 1 111111 31 11.5 42 13 12 77 11.5 4.2 7 1111 4.11 1.337 1511 3.11 11.4 2.2 1.3411 5211 13 14 22 24 32 44 7.2 11.11 11 so 3.11 1.327 15 4.3 7.7 1.2 1,342 7511 111 11 211 111 23 521 9.4

1.8 3.0 L327 III) 5.5 (1.0 1.11 L345 I0 3 8.: 4| ll J (18 9.4 3.2 ll) NU 3.!) L327 l-il) 7.3 4.3 25 L347 I051) 32 7.) 45 ll 8 (11) NJ 7 '1 11.3541 11113111 13111 114.41 (431 (131 ()1 11141 (11, 11 (117 11 so 3.11 1.327 11111 7.11 4.1 2.11 1.1511 11511 411 7.7 1111 11 x 51: 5.2 4.11

(1.3541 1113111 1431 18.15) 15111 (121 191 111111 111.91 11.111 12 so 311 1.327 11111 5.5 11.11 2.1 1353 1111111 2 1 s: 3s 11 11 711 3.1 1 3.5 13 1 111 3.11 1.327 1 111 11.3 5.2 2.7 1 .1511 1111111 33 7 1 42 12 1 711 5.11 3.s 14 1111 3.11 1.327 2111 5.2 11.3 11.7 1.3411 1711 is 711 24 11 15 111 1 7.11 11.5 l5 fill 31) 1.3 l-l ISH 5.5 4.3- Kl unmeasurahle due to large unucnncss of thickness 3.5 H1 (15 2.11 l 13 l51| 11.11 4.11 l l1) unmcasurahlc due to large unewnness of thickness 0.5 I7 91) 3.1% L337 3.) h i (14 L114) 550 I? I Z2 Z3 35 4H 7.5 7.2 114 11111 3.11 1.3311 1511 4.11 71 7 5 1.14 11111 14 I 25 21 11 411 7 5 7,4

l Il\'\sll\'.\- of thickness \\.1\ measured h lticans 111 1 dial gauge and 1s 1'\ \lt"-\L'1l lo the l'11ll11\\111 lmlnula Maximum thicknes Minimum 'll1lLl\llg' I111) 1'1 1 A\cr:\gc thickness Notes:

I. The stretching ratio in the extruding direction shows the maximum stabilized stretching ratio under the conditions described in the above Table.

2. The heat shrinkage shows the percentage variation of the dimension of the film after heated in the air at l5()C. for 2 hours.

3. MD and TD in the Table show the longitudinal direction (machine direction) and transversal direction respectively.

EXAMPLE 3 A substantially amorphous film obtained in Example I, was at the same time biaxially stretched to 2 times X 2 times at 80C by means of the same apparatus as Table 3 Stretching temperature IRH mo Wt) l-'-5 \alue (kg/mm! 2. 37 2h Tensile strength (Kg/mm") 33 5 Foo 35.2 lilongation at break ('3 i l8 12 Ill 12) l 12] l3) Initial modulus of Ntlt) )Zll )tll) elasticity (Kg/mm Heat shrinkage b heat- 7.5 (1.0 3.)

treatment at l5l|(. i I for 2 hours Note, \alue in the parentheses are those in the II) direction.

EXAMPLE 4 A substantially amorphous film obtained in Example I was at first stretched to 2 times in the perpendicular direction to the extruding direction at 85C and then stretched at 85C in ratios shown in Table 4 in the extruding direction, by means of the same apparatus as in Example I (first step stretching). Further. each resulting film was stretched to 5.5 times in total in the extruding direction at l4tlC (second step stretching). then subjected to crystallization at 200C. for l minute. under tension. and heat-set.

The physical properties of the resulting films are shown in Table 4.

Table 4 Second step stretching ratio 2 l5 3 345 in the stretch- F-5 \aluc ing direetion 27 2X 27. 5.5

at 5.5 times Tensile strength (Kg/mm") 35 38 37 33 lilongation at break 1 I I) 15 J ltl Initial modulus of clasticit )Ull )htl U31) 83H (Kg/mm'-') Heat shrinkage h heat treatment at .11 L8 L8 1,8

l5tlC for 1 hrs ('1') Tensile in the stretchstrength ing direction ltLtl l I. 1 L8 l l.5

(Kg/mm") at 2 times Densit after lst step stretching L313 L324 L325 L317 EXAMPLE 5 in Ex'im le l to roduce l film h'ivin l densit I of l t t g A substantially amorphous film obtained in Example f S 18!" ln S I lmW'lS urth i t i ir t m l was at tn'st stretched m the transversal direction in l UlTlZlXlil 'S l'C C C U -4- tmet l In C e t h 5 T N 1 l t the first step stretching and then stretched m the long!- T'l urs S VI 11 C SCLOHC S S TC C In m th th g d tudinal direction In two divided steps in the second step "Sl "tl ['0 er IDS U C 1111s In C SCCOll ,e I f p h T M 1 stretching under the conditions shown in Table 57 lhen S I Ll l)l'l'll'eSU\Nnll1 l 9..

u g i each resulting film was heat-set at 2 l(lC. The physical SS properties of the films thus obtained are shown in Table 5.

Table 5 First step stretching Second step stretching Ph \sical properties of film lst step 3nd step hick- Initial F-5 \alue Tensile Heat Tensile Tcmpei Ratio Film Tem- Ratio Tem- Ratio Total ness modulus in MD strength shrinkage strength .lllltC densit} peraperaratio (at of elas direction in MD in MD in TD tgiem l ttlre turc X ticit tkg/mm 'l direction direction direction in MD (kg/mm) ('4 l (kg/mm) direction (I) 12) (kg/mm! 3 Hill 14 L525 1.) l7 an 5 I15 42 NI ".0 J

IHIMPC |Hl5ll(' NI )7 l 327 13H 4} l5ll l,5 (1.45 8,7 llllll 37 53 L5 ll) so ,1 o Isis no 47 iii! L4 (15s 7Y5 [H65 36 4x If to w 11 427 Ht) *H tit! if no 7*! mo 28 i I45 ll 'l able 5 Continued First step stretching Second step stretching Physical properties of film lst step 2nd step initial F-5 value Tensile Heat 'l'ensile hickl'emper- Ratio Film Tcm- Ratio fem- Ratio l'otal ness modulus in MD strength shrinkage strength aturc density peraperaratio (]J.l ol elasdirection in MD in MD in Tl) (g/Clt'l"! tun: ture X ticity (kg/mm") direction direction direction In MD (kg/mm) ('4 (kg/mm") direction (I) (2) (k /mm) (3) 7K 3.6 1.328 I30 5.3 I50 1.2 (1.36 (no UN) 28 37 H ll 80 3.0 l 328 l30 5.0 I50 1.2 (1,0 7.0 1000 32 43 H I:

Notes:

EXAMPLE 6 Copolymer A-40 having it) mol "/r of terephthalic acid copolymerized (having an intrinisc viscosity of 0.650 as measured in the same measuring manner) was extruded at 280C from T die onto a casting roll cooled to 60C to obtain a substantially amorphous film having a thickness of I 1. and a density of 1.301. which was successively led to a tenter apparatus and stretched there at 80C to 3.2 times the original width in the transversal direction. The density of film at this time was 1.326. Next. the resulting film was stretched at l60C to 5.8 times the length after the first step stretching in the longitudinal direction by means of the longitudinally stretching apparatus of Example 2. and then heat-treated at 2l0C. The physical properties of the resulting film were as follows:

'l'hickcncss: 5.4 u l ncvcnncss of thickness in Tl): 2.0"; Density: IJSI initial modulus of elasticity: 9-10 Kg/nnn F-i (MD): 27 Kg/mm F- (TD) 55 kg/mm Tensile strength (MD): 36 Kg/nnn" Tensile strength I l0): l3 Kg/mm Elongation at break lMD): l-ll Elongation at break (TD): 70%

Heat shrinkage (MD): 41));

Heat shrinkage ('l D): 2,8;

Comparative Experiment 1 A substantially amorphous film obtained in Example l (200 a thick) was biaxially stretched at the same time longitudinally and laterally to 3.75 times at 95C by means of a biaxially stretching machine, and then heatset at 2l0C. The physical properties of the resulting film were as follows:

Thickness: Hp

Density: L353 (g/cm") F-F value I41 (Kg/mm") Yield strength: INS IKg/nim) Tensile strength. 2-8.5 [Kg/Him") Initial modulus of elasticity 500 (Kg/mm-l Flongation at break 4!) ('r Heat shrinkage at lFlltl' Z l) (W) for 2 hours Comparative Evpcrimcnt Z Substantially amorphous film [having a thickness of W0 1 and an unevenness ofthickness of 4% obtained hi i in Example 2 was heat-treated at 180C for 30 seconds on a heated roll and then roll-stretched at l50C to 4.7 times the original length in the longitudinal direction. The physical properties of the resulting film were as follows:

As evident from the above Examples. the films of the present invention. as the result of improvement of known processes as well as establishment of conditions suitable to the polymers of the present invention. have remarkably improved mechanical properties as compared with conventional films strengthened in the longitudinal direction (ie. in the lengthy direction).

Namely. the films of the present invention are biaxi ally oriented films of high grade having such strengths that the initial modulus of elasticity is in the range of 800 to L500 KgIcm the F-5 value is in the range of 20 to 50 Kg/mm (preferably 23 Kg/mm or higher); and the tensile strength is in the range of 30 to Kg/- mm while the tensile strength in the lateral direction (i.e.. in the width direction) is in the range of 8 to 20 Kg/mm. Further the films of the present invention have also a great advantage in that they are superior in the dimensional stability at the time of heating. Furthermore. they have also such an important feature of superior form-ability of thin foil that. as shown in the above Examples. films having a thickness of 4 6 a and having the above-mentioned physical properties can be prepared in stabilized manner. In view of the abovemcntioned features. the films of the present invention are useful for magnetic recording tape. soundrccording tape. video tape. typewriter ribbon. measur ing tape. adhesive tape. film for gold or silver threads. film for stamping foil. etc

For magnetic tapes. it is in general considered to be better in quality that the tapes have a greater strength in the longitudinal direction. a certain extent of strength in the transvcrsal direction. a smaller unevenness of thickness and a smaller heat shrinkability 'lhc 1 3 films of the present invention satisfy all of the abovementioned conditions and hence are most suitable for magnetic tapes.

What is claimed is:

l. A hiasially stretched l'ilm suitable for use as a tape for magnetic recording which is a hiaxially stretched polyethylene-1.Z-diphenoxyethane-4A'-dicarboxylatc homopolymer film having a longitudinal F-S value of 20 Kg/nnn" to 50 Kg/mm. a longitudinal tensile strength of 30 lsg/mm" to 65 Kg/mm a longitudinal initial modulus of elasticity of 800 Kg/mm to 1500 Kg/mnfl a transversal tensile strength of 8 Kg/mm to 20 Kg/mm and an unevenness of thickness of or less. which is produced by first stretching at a temperature in the range of 70-100C a substantially amorphous polyethylene-1.2-diphenoxyethane-4 4'- dicarboxylate layer whereby there is produced an orientation at least in the direction perpendicular to the extrusion direction and a film density after stretching in the range of 1.315 to 1.335 in said first stretching, and thereafter subjecting the thus-treated film to a second stretching step in the extrusion direction at a temperature of 100C to 100C to yield a total stretching ratio of 5 to 8 times the original length in said second stretching step.

2. Biaxially stretched films according to claim 1 wherein said longitudinal F-S value is 23 Kg/mm to 50 14 Kg/mm" said longitudinal tensile strength is 33 Kg/mm" to 65 Kg/mm. said longitudinal initial modulus of elasticity is 900 Kg/mm to 1500 ls'g/mm and said transversal tensile strength is 10 Kg/mm to 20 Kg/mmi 3. Biaxially stretched films according to claim 2 wherein said longitudinal F-5 value is 30 Kg/mm to 50 Kg/mm.

4. Biaxially stretched films according to claim 1 wherein the thickness of film is in the range of 4 u to 6 ,u.

5. Biaxially stretched films according to claim 1 wherein heat-shrinkage of the films after the processing at 150C for 2 hours is in the range of l to 47:.

6. A method for producing strengthened polyethylene-1 ,2-diphenoxyethane-4A'-dicarboxylate films which comprises stretching at a temperature in the range of lO0C a substantially amorphous polyethylene- 1.2-diphenoxyethane-4,4'-dicarboxylate layer to give an orientation at least in the direction perpendicular to the extrusion direction a stretching ratio in the range of 2-4 times the original width and a film density after stretching of 1.335 or less in the first step stretching, and then stretching in the extrusion direction at a temperature of C to 200C to give a total stretching ratio of 5 8 times the original length in the second step stretching. 

1. ABIAXIALLY STRECHED FILM SUITABLE FOR USE AS A TAPE FOR MAGNETIC RECORDING WHICH IS A BIAXIALLY STRECHED POLYETHYLENE-1,2-DIPHENOXYETHANE-4,4'' -DICARBOXYLATE HOMOPOLYMER FILM HAVING A LONGITUDINAL F-5 VALUE OF 200 KG MM2 TO 50 KG/MM2, A LONGITUDINAL TENSILE STRENGTH OF 30 KG/MM2 TO 65 KG/MM2, A LONGITUDINAL INITAL MODULUS OF ELASTICITY OF 800 KG/MM2 TO 1500 KG/MM2, A TRANSVERSAL TENSILE STRENGTH OF 8 KG/MM2 TO 20 KG/MM2 AND AN UNEVENNESS OF THICKNESS OF 5% OR LESS WHICH IS PRODUCED BY FIRST STRECHING AT A TEMPERATURE IN THE RANGE OF 70*--100*C A SUBSTANTIALLY AMORPHOUS POLYETHYLENE-1,2-DIPHENOXYETHANE-4,4''-DICARBOXYLATE LAYER WHEREBY THERE IS PRODUCED AN ORIENTATION AT LEAST IN THE DIRECTION PERPENDICULAR TO THE EXTRUSION DIRECTION AND A FILM DENSITY AFTER STRETCHING IN THE RANGE OF 1.315 TO 1.335 IN SAID FIRST STRETCHING, AND THEREAFTER SUBJECTING THE THUS-TREATED FILM TO A SECOND STRETCHING STEP IN THE EXTRUSION DIRECTION AT A TEMPERATURE OF 100*C TO 100*C TO YIELD A TOTAL STERTCHING RATIO OF 5 TO 8 TIMES THE ORIGINAL LENGTH IN SAID SECOND STRETCHING STEP.
 2. Biaxially stretched films according to claim 1 wherein said longitudinal F-5 value is 23 Kg/mm2 to 50 Kg/mm2 , said longitudinal tensile strength is 33 Kg/mm2 to 65 Kg/mm2, said longitudinal initial modulus of elasticity is 900 Kg/mm2 to 1500 Kg/mm2 and said transversal tensile strength is 10 Kg/mm2 to 20 Kg/mm2.
 3. Biaxially stretched films according to claim 2 wherein said longitudinal F-5 value is 30 Kg/mm2 to 50 Kg/mm2.
 4. Biaxially stretched films according to claim 1 wherein the thickness of film is in the range of 4 Mu to 6 Mu .
 5. Biaxially stretched films according to claim 1 wherein heat-shrinkage of the films after the processing at 150*C for 2 hours is in the range of 1 to 4%.
 6. A METHOD FOR PRODUCING STRENGTHENED POLYETHYLENE-1,2DIPHENOXYETHANE-4,4'' -DICARBOXYLATE FILMS WHICH COMPRISES STRETCHING AT A TEMPERATURE IN THE RANGE OF 70*-100*C A SUBSTANTIALLY AMORPHOUS POLYETHYLENE-1,2-DIPHENOXYETHANE-4,4''DICARBOXYLATE LAYER TO GIVE AN ORIENTATION AT LEAST IN THE DIRETION PERPENDICULAR TO THE EXTRUSION DIRECTION A STRETCHING RATIO IN THE RANGE OF 2-4 TIMES THE ORIGINAL WIDTH AND A FILM DENSITY AFTER STRETCHING OF 1:335 OR LESS IN THE FIRST STEP STRETCHING, A AND THEN STRETCHING IN THE EXTRUSION DIRECTION AT A TEMPERATURE OF 100*C TO 200*C TO GIVE A TOTAL STRETCHING RATIO OF 5 - 8 TIMES THE ORIGINAL LENGTH IN THE SECOND STEP STRETCHING. 